The high resolution attainable with transmission electron microscopy makes this an outstanding technique for examining the microstructure of materials. The direct examination of materials by transmission electron microscopy requires that the specimen to be examined is transparent to electrons. Consequently the thickness of the specimens must be restricted to 100 to 200 nm.
It has therefore been necessary to develop methods for preparing thin specimens of materials that have widely varying mechanical and chemical properties. Soft materials such as biological specimens, may be prepared by microtoming, although difficulty is sometimes encountered when hard particles are present.
For some metals, semiconductors, and other inorganic materials, chemical etching and electrolytic techniques are suitable. In one widely used method the material to be thinned is placed in a jet etching tank and the etching process observed through a lens with a light source behind the specimen.
The etching action of the jet is continued until perforation of the specimen occurs. Since the etching action is stronger at the centre of the jet than at its periphery, perforation starts at the centre and spreads towards the periphery. Thus the etching process is immediately arrested when perforation occurs by flushing the specimen with an inhibiting wash, leaving adjacent areas around the perforation which are usually sufficiently thin to allow micrographs to be taken during examination under the electron microscope.
Difficulties arise with such chemical etching methods when materials are not homogeneous. Thus preferential etching may occur, second phases may be leached out, and in semiconductors p-type material may etch at a different rate from n-type material. Even when a material can be controllably etched, the etchant may form a contaminating layer on the surface.
For materials for which suitable chemical etchants do not exist, such as some glasses, ceramics, and geological specimens, various mechanical preparation techniques have been tried. The specimen may be crushed and fine slivers selected, or thin sections may be produced by very careful mechanical polishing. These operations require considerable skill and can generally not be applied to brittle granular materials with voids.
However a large variety of materials which do not lend themselves to chemical treatment, are thinned by ion erosion. Ion erosion has proved an increasingly valuable tool to electron microscopists especially those engaged in examining classes of material such as ceramics, impurity-doped semiconductors and alloys. These materials are difficult to etch chemically or can contain constituents that etch at widely differing rates. Where chemical or electrolytic etching are possible however the liquid techniques are still favoured because the equipment is generally less costly, the etching rate is faster and of course there is no radiation damage although specimens may suffer chemical contamination.
Because of the relatively slow ion etching rates materials are commonly prethinned by mechanical or chemical means or a combination thereof. Some specimens may be prethinned to say 25 .mu.m quite readily, in other cases a considerable amount of skill and patience may be required. In other cases specimens cannot be prethinned to less than a few hundred micrometers because the material may be friable and crumble or inclusions may be lost from the matrix.
In typical known ion thinning equipment ion beams of about 2 mm diameter from two sources impinge centrally on either side of a specimen. A hole or perforation is allowed to form in the specimen by the ion beam, which acts in a similar way to the chemical jet during chemical etching, when the ion beam is immediately turned off to leave adjacent thin areas around the perforation transparent to electrons. The ion current from each source may be about 100 .mu.A, and the cathodes of the source from which beams emerge are at a distance of about 4 centimeters from the specimen. Under these conditions the thinning rate to penetration for most specimens is in the range of 1 to 10 .mu.m/hr and for many specimens the rate is near 3 .mu.m/hr.
A typical sample thickness is 50 .mu.m. Assuming therefore an ion thinning rate of 3 .mu.m/hr it would take 16 hours to thin the specimen to penetration by ion erosion.
Some materials 125 micrometers thick, may take two to three days to thin by ion erosion, although in one case an etch rate of 15 .mu.m/hr has been claimed. Even for this relatively fast rate, chemical etching is one or two orders of magnitude faster.
The etch rate of course depends on the material, and a generally accepted rate for ceramics is 1 to 2 .mu.m/hr with a glancing angle of 20.degree. and an ion current density of 200 .mu.A/cm.sup.2. Where possible therefore samples are prethinned to 20 to 30 .mu.m although porous or friable materials are generally thicker.